Yum transaction history.

Since RHEL 6, a transaction history has been added to the yum command.
This feature allows you to precisely know which packages have been installed on a server and in which order.
In addition, you can undo or redo any of the previous package installations.
Finally, by comparing the rpmdb version of two servers, you can verify if the exact same packages have been installed on both of them, which can be useful when moving from development to production environments.
To get into the details, check the tutorial about using the yum transaction history.